This invention relates to improvements in systems that measure the quantity of solute in a flow of carrier fluid with a detector having a chamber into which the fluids may be injected and means for producing an electrical signal corresponding to the average value of a given characteristic of the fluids in the chamber. The combination of solute and carrier fluids is referred to as "sample fluid". When no solute is present in the sample fluid passing through the chamber, the electrical signal has what is known as a "baseline value", and when a concentration of solute is present in the sample fluid, the electrical signal changes from the baseline value to form a peak. The area between the peak and the baseline value corresponds to the quantity of the concentration of solute that has passed through the chamber. For various reasons, the baseline value drifts slowly with time so as to complicate measurement of the area.
This problem has been overcome by an invention of John S. Craven and David E. Clouser, set forth in their U.S. patent application entitled "Modulated Fluid Detector", Ser. No. 730,559, filed on Oct. 7, 1976, now U.S. Pat. No. 4,254,654. In accordance with that invention, sample fluid and reference fluid are alternately injected into the chamber of the detector a number of times during the peak in the electrical signal caused by the solute. The reference fluid may be carrier fluid or fluid that has the same value of the characteristic to which the detector responds as the carrier fluid, and that value is less or greater than the value of the characteristic of any solute to be analyzed. The electrical signal thus contains alternations betwen the value of the electrical peak signal referred to and the baseline value. The signal was synchronously detected so as to produce an output signal corresponding to the difference between the peak and the baseline values, and inasmuch as the baseline value is practically the same during each half of the alternation, it is eliminated by subtraction.
In applying the invention of the above patent application to a thermal conductivity or TC detector, alternate flows of sample gas, which is comprised of carrier gas having concentrations of the solute gas to be analyzed, and reference gas, which may be carrier gas or other gas having the same thermal conductivity, are made to pass through a single chamber in which a filament is suspended. The output signal of the detector is the voltage required to keep the filament at a constant temperature or resistance as it is cooled by heat flowing from the filament to the wall of the chamber. The output signal thus depends on the thermal conductivity of the gas in the chamber. When the chamber is filled with reference gas, the voltage has a baseline value; and when the chamber is filled with sample gas, the voltage has a value depending on the thermal conductivity of the sample gas--the greater its thermal conductivity, the greater is the flow of heat from the suspended filament to the wall of the chamber and the greater, therefore, is the voltage output of the detector.
As originally conceived, the flow rates of the sample and reference gases were sufficiently large that the chamber was filled with sample gas during nearly all of one half-cycle of the alternate flow and with reference gas during nearly all of the next half-cycle. The synchronous detector was operated in phase with the alternation of flow of sample and reference gases so as to produce an output signal corresponding to the difference between the detector signal occurring when the chamber was full of sample gas and the detector signal occurring when the chamber was full of reference gas. Whereas this virtually eliminated the effects of variation in the value of the baseline, it was found that flow noise was introduced into the output signal whenever there was a change in the difference between the flows of sample and reference gases. This problem was met by another invention of John S. Craven and David E. Clouser set forth in their U.S. Pat. No. 4,185,490 in which the flows of sample and reference gas were such that each gas just filled the chamber at the end of its half-cycle of flow so as to produce a detector signal having a maximum value at the end of one half-cycle and a minimum value at the end of the other half-cycle. In addition, the synchronous detector was operated 90.degree. out of phase with the gas flow so as to derive an output signal corresponding to the difference between the maximum and minimum values.
Whereas this type of operation reduces the flow noise to a minimum, it cuts the amount of sample gas introduced into the chamber in half and reduces the signal that can be attained by a like amount. Furthermore, as long as the gases are flowing through the chamber when a measurement is being taken by the synchronous detector, differences in heat capacities or the diffusion coefficients of the sample and references gases may have adverse effects on the measurements.